Is Champion Mastery score irrelevant of ELO?

Solution 1:

Yes, that is correct.

Champion Mastery takes into account the position you've played and compares your Score (Overall Parameters, not just K/D/A, however details are unknown) with the score of other players playing the same champion in the same role.

It is incorrect however assuming that:

when a silver player scores S+ does that mean that he performed as well as a diamond player?

This is incorrect as even if the Diamond player can only play that champion with a B- rating, and you succeeded with S+ rating, despite you having a better performance, it doesn't necessarly mean that you are better than him (since he is facing stronger opponents after all), it means you perform better at your ELO than he does at his.

Short Awnser, yes, it is independent of MMR